This recording is a real-world example of a 3G-ALE FLSU (Fast Link Setup) asynchronous call copied on 15062 KHz/USB: although in 3G networks synchronous calls is the preferred mode, async call might be used if the called (or the caller) station may not have achieved net synchronisation.
The async call of the fast link setup protocol begins with the LBT (listen before transmit) for at least one dwell period, followed by the transmission of 1.35N (nearest integer value) Async Request PDUs on the requested link frequency, where N is the number of channels in the scan list, and 1.35 is the duration of each dwell period in seconds. The async call procedure ends with a single LFSU Request (Fig. 1).
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| Fig. 1 |
As in the previous post transmission consists of several BW-5 bursts sent multiple times (Fig. 2)
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| Fig. 2 |
Looking at the 50-bit payloads in Fig. 3, the PDU of type "011" is sent 13 times, followed by a single PDU of type "000": since PDU type "011" identifies the Async_FLSU_Req PDU, and "000" the FLSU_Request PDU, the sample exactly matches the async call procedure as illustrated above. It's worth noting that since there are 13 Async_ FLSU_Request PDUs, the number of the channels for this network is equal to 9.
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| Fig. 3 |
Quoting from STANAG-4538 "Transmitting 1.35N Async_FLSU_Request PDUs guarantees that all other scanning stations will scan the calling channel during the async call, even under the worstcase time of day (current time) offset conditions. If the time of day offset can be estimated more accurately, fewer than 1.35N Async_FLSU_Request PDUs may be sent to capture the desired station.
Since the address of the called station(s) is contained in the Async_FLSU_Req PDU, all stations that are not included in the call are free to resume scanning. Called station(s) that receive one of the asynchronous FLSU PDUs stop scanning and wait for the normal FLSU_Request PDU, which is sent immediately after the final Async_FLSU_Request PDU. The maximum wait duration is approximately equal to 1.35(N + 1) seconds, where N is the number of frequencies in the scan list".
Since the address of the called station(s) is contained in the Async_FLSU_Req PDU, all stations that are not included in the call are free to resume scanning. Called station(s) that receive one of the asynchronous FLSU PDUs stop scanning and wait for the normal FLSU_Request PDU, which is sent immediately after the final Async_FLSU_Request PDU. The maximum wait duration is approximately equal to 1.35(N + 1) seconds, where N is the number of frequencies in the scan list".
STANAG-4538 5.3 Annex C "Protocol data units" helps to parse the call:
001 00 0000100001 0000000100 0 0 011 111111 000010 10011000 [Async_FLSU_Req]
001 00 0000100001 0000000100 0 0 000 111111 000010 00001110 [FLSU_Request]
Protocol Field
001 = FLSU PDU
001 = FLSU PDU
Priority
indicates the priority level of the traffic (if any) that the sender of the PDU intends to send on the link once it is established. 0 = highest priority
Destination Address 0000100001 is the address of the PU to which this PDU is being sent
Source Address0000000100 is address of the PU that is sending this PDU
indicates the priority level of the traffic (if any) that the sender of the PDU intends to send on the link once it is established. 0 = highest priority
Destination Address 0000100001 is the address of the PU to which this PDU is being sent
Source Address0000000100 is address of the PU that is sending this PDU
Address Type
Address type indicates if the 10 bit destination address represents a multicast group or an individual address. In this case 0 = Point-To-Point: destination is an individual address
XN (Cross Net)
The XN field indicates that the destination is in the same network as the caller when set to 0. When set to 1, the destination is in a different network
The XN field indicates that the destination is in the same network as the caller when set to 0. When set to 1, the destination is in a different network
PDU type
3 (011) = Asynchronous FLSU_REQUEST
0 (000) = REQUEST 2Way: a “request with acknowledgment”
3 (011) = Asynchronous FLSU_REQUEST
0 (000) = REQUEST 2Way: a “request with acknowledgment”
Argument1
in case of PDU type = REQUEST specifies the channel used for traffic. A default value of 63 ("111111") specifies that the current calling channel shall be used for traffic
in case of PDU type = REQUEST specifies the channel used for traffic. A default value of 63 ("111111") specifies that the current calling channel shall be used for traffic
Argument2
in case of PDU type = REQUEST specifies the traffic type, in this case 2 (010) = digital voice
in case of PDU type = REQUEST specifies the traffic type, in this case 2 (010) = digital voice
CRC
The CRC field contains an 8-bit cyclic redundancy check (CRC) for error detection, computed over the preceding 42 bits of the PDU. The generator polynomial is X8 + X7 + X4 + X3 + X + 1.
The CRC field contains an 8-bit cyclic redundancy check (CRC) for error detection, computed over the preceding 42 bits of the PDU. The generator polynomial is X8 + X7 + X4 + X3 + X + 1.
As a finale note, it's interesting to see in Fig. 5 that the on-the-air symbols of the async call have a 7296 bit length period, ie exactly 2432 PSK-8 symbols, just because of the (nearly) uniformity of the payloads while in the previous post the payloads' rotation probably caused the 768 bit period.
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| Fig. 4 |
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| Fig. 5 |
